1. Field of the Invention
The present invention relates to a material constituted by ultrafine metallic particles and by ultrafine oxide particles, a process for its preparation, and its utilization, particularly for catalysis.
2. Description of the Related Art
Materials constituted by fine metallic particles are known for diverse applications, particularly in catalysis. Various processes for preparing them have been described.
L. K. Kurihara et al. (Nanostructured Materials, Vol. 5, No. 6, 607-613, 1995) describe the preparation of nanometric metal particles of a metal by a so-called xe2x80x9cpolyolxe2x80x9d process, used for various easily reducible metals such as, for example, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Sn, Re, W, Pt, alone or as a mixture. Such a process consists of dissolving or suspending a precursor of the said metal in a polyol, then heating under reflux to obtain a precipitate of metallic particles. The particles obtained can be nanometric. They are polymetallic when the reaction medium contains salts of different metals. The production of nanometric Cuxe2x80x94Co particles by the polyol process is described in L. K. Kurihara et al., J. Mat. Res., Vol. 10, No. 6, June 1995.
Nanometric particles containing aluminum can be prepared by reducing a salt with LiAlH4. Thus the preparation of nanocrystalline Ti/Al and Ni/Al particles from a chloride of the corresponding metal in the presence of LiAlH4 is described by J. A. Haber et al., Advanced Materials, 1996, 8, No. 2. The reaction mechanism of this process is the following:
Mn++nLiAlH4xe2x86x92nLi++M+nAlH3+n/2H2xe2x80x2nAlH3xe2x86x923n/2H2xe2x80x2,
the content of Al having an upper limit corresponding to the valence of the associated metal.
The preparation of nanometric intermetallic particles by means of borohydrides is furthermore known. S. T. Schwab et al., Materials Science and Engineering, A204 (1995) 197-200, describe the preparation of intermetallic particles by the reduction of a mixture of salts of different metals by a borohydride (for example, TiCl4+AlCl3+lithium triethyl borohydride). This process enables intermetallic particles to be obtained without limitations on the content of the respective metals. It requires a heat treatment at 1,000xc2x0 C. to decompose the products obtained by the reaction of the salts with the borohydride and to obtain the metallic forms.
The preparation is likewise known of fine monometallic particles by the reaction of a metal salt with sodium hydride in solution in an organic solvent and in the presence of an alcoholate. Such a process has been described for nickel particles and for zinc particles (Paul Caubere et al., J. Am. Chem Soc., 1982, 104, 7130; P. Gallezot, C. Leclercq, Y. Fort, P. Caubxc3xa9re, Journal of Molecular Catalysis, 93 (1994) 79 83, pp. 80-83; Brunet et al., Journal of Organic Chemistry 1980, Vol. 45, pp. 1937-1945). The particles obtained have a crystallite dimension of nm order and are particularly useful as a catalyst for heterogeneous catalysts. When they are prepared in the presence of a phosphine or 2,2xe2x80x2-bipyridine ligand, they lose their reducing properties and behave as a coupling agent (Lourak et al., Journal of Organic Chemistry, 1989, Vol. 54, pp. 4840-4844).
An analogous process has likewise been described for the preparation of monometallic particles of Pd, which can be used as a catalyst for the hydrogenation of acetylene (J. J. Brunet and P. Caubxc3xa9re, J. Org. Chem., 1984, 49, 4058-4060).
The usefulness of ultrafine polymetallic particles as catalysts for various reactions is known. The French patent application No. 97.11814 describes a process which enables ultrafine polymetallic particles to be obtained containing aluminum having good purity, with any given aluminum content. The process consists of reducing a mixture of salts in solution in an organic solvent by means of a hydride of an alkali metal or alkaline earth metal, at a temperature lower than or equal to the reflux temperature of the solvent, the mixture of salts in solution comprising at least one salt of a metal having a standard oxidation potential E0(Mn+/M) at 25xc2x0 C. greater than xe2x88x921.18 V. The material obtained by this process is constituted by nanometric polymetallic particles which have a mean crystallite dimension less than 4 nm, in which the various metals present are intimately associated, and which are constituted by at least one metal chosen from the group constituted by Ni, Co, Fe, Cu, Zn, Cd, Cr, Mn, Pd, Ag, Pt, Au, Bi and Sb, and at least one metal chosen from the group constituted by V, Zr, Ce, Ti, Hf and Al. Such a material can be used as a catalyst for diverse reactions, particularly for the hydrogenation of olefins and for the direct coupling of aromatic halogenated derivatives.
The present inventors have now found a process permitting the preparation of metallic particles which have improved catalytic performance.
The present invention thus has as its object a material constituted by ultrafine metallic particles and by ultrafine oxide particles, a process for its preparation, and also its utilization as a catalyst.
The material which is the object of the present invention comprises at least one metallic element M of degree of oxidation 0 having catalytic properties, and at least one metallic element Mxe2x80x2 having a standard oxidation potential less than that of the element M, at least a portion of the Mxe2x80x2 atoms being in an oxidized form; and it is characterized in that:
it is in the form of particles having a mean dimension less than 50 nm, at least 80% by number of the particles having a mean dimension less than 10 nm;
a particle of the material is constituted by
at least one metallic element M of degree of oxidation 0, or by
at least one metallic element Mxe2x80x2 in oxidized form, or by
at least one metallic element Mxe2x80x2 of degree of oxidation 0, or by
the combination of at least two species chosen from the three preceding species, it being understood that the mean content of the element M of degree of oxidation 0 in the material is greater than 25% by number of atoms with respect to the whole of the material, the mean total content of the element Mxe2x80x2 is at most equal to 75% by number of atoms with respect to the whole of the material, and the mean content of the element Mxe2x80x2 in the oxidized form is greater than 10% by number of atoms with respect to the total content of the element Mxe2x80x2.
A metal Mxe2x80x2 which has a standard oxidation potential less than that of a metal M is a metal which is easier to oxidize than the metal M.
An element M or Mxe2x80x2 of degree of oxidation 0 will hereinafter be respectively denoted by M(0) or Mxe2x80x2(0).
A material according to the invention can furthermore contain the element M in oxidized form, under the condition that the proportion of the oxidized element M is not greater than 10% by number of atoms with respect to the total quantity of the element M.
The element M is a metal having catalytic properties. It is advantageously chosen from among Pd, Pt, Rh, Ir, Ni, Co, V, Mo, Zn, Cd, Cu, Ag, Au and Fe.
The element Mxe2x80x2 is chosen from among V, Zr, Ce, Ti, Hf, Al, Ni, Co, Fe, Ru, Cu, Zn, Cd, Cr, Mn, Bi, Sb and Si. The choice of the element Mxe2x80x2 is made such that the standard oxidation potential of the selected element Mxe2x80x2 is less than that of the element M with which it is associated. The pairs (M, Mxe2x80x2) such as (Ni, Fe), (Ni, Al), (Pd, Cu), (Pd, Ni) or (Pd, Al) are particularly preferred.
When M is Ni, Co, or Fe, the mean dimension of the particles constituting the material of the invention is xe2x89xa610 nm, 80% by number of the particles having a mean dimension less than 5 nm.
The material of the present invention can be obtained by subjecting to an oxidation, polymetallic particles containing at least one element M(0) having catalytic properties and at least one element Mxe2x80x2(0) having a lower standard oxidation potential than that of the metal M, the said particles having a mean dimension less than 50 nm, preferably less than 10 nm. These particles are denoted hereinafter by xe2x80x9c[M(0), Mxe2x80x2(0)] polymetallic particlesxe2x80x9d. The oxidation is effected by means of oxygen or water. It is particularly advantageous to use the oxygen contained in the air.
The total or partial oxidation of the element Mxe2x80x2 contained in the [M(0), Mxe2x80x2(0)] polymetallic particles brings about the enrichment of the particles in the element M(0) (particularly at the surface of the particles) and the reduction of the mean dimension of the particles. In the obtained particles, the element M(0) and possibly the element Mxe2x80x2(0) are intimately associated with an oxidized form of the element Mxe2x80x2. According to the oxidizing agent used, the oxidized form of the element Mxe2x80x2 is an oxide or a hydroxide.
The material of the invention should not contain more than 10% by number of atoms of the element M in the oxidized form, with respect to the total content of the element M. To obtain this result, it is recommended to use the oxidizing agent in a stoichiometric quantity with respect to the element Mxe2x80x2 to be oxidized, if the total oxidation of Mxe2x80x2 is desired, or in a sub-stoichiometric quantity if a partial oxidation of Mxe2x80x2 is desired. In any case, when the element M is a noble metal chosen from among Au, Pd, Pt, it is recommended to use an excess of oxidizing agent.
The [M(0), Mxe2x80x2(0)] polymetallic particles subjected to oxidation can be obtained by various processes.
In a preferred embodiment of the invention, the [M(0), Mxe2x80x2(0)] polymetallic particles are obtained by the reduction of a mixture of salts of the metals M and Mxe2x80x2 in solution in an organic solvent by means of a hydride of an alkali metal or an alkaline earth hydride. The metal M having a catalytic activity can be chosen from among Pd, Pt, Rh, Ir, Ni, Co, V, Mo, Zn, Cd, Cu, Ag, Au and Fe. The element Mxe2x80x2 is chosen from among V, Zr, Ce, Ti, Hf, Al, Ni, Co, Fe, Ru, Cu, Zn, Cd, Cr, Mn, Bi, Sb and Si, it being understood that the element Mxe2x80x2 is chosen such that its standard oxidation potential is less than that of the element M with which it is associated. The salt of the metal M and the salt of the metal Mxe2x80x2 can be chosen from among the chlorides, bromides, iodides, acetates, acetylacetonates and alcoholates. The proportions of the various salts used for the reaction are chosen as a function of the composition desired for the polymetallic particles, it being understood that the proportions are conserved after reduction by the hydride. When commercial hydrated salts are used, it is preferable to grind them finely before drying them under vacuum, in order to prevent any decomposition. The hydride can be chosen from among LiH, NaH, KH, CaH2 and MgH2. NaH can advantageously be used in the form of a commercial solution, 65% by weight in mineral oil, if necessary after washing with an aprotic organic solvent. LiH can be utilized in the form of a powder such as is available commercially. Such a process of obtaining [M(0), Mxe2x80x2(0)] polymetallic particles is described in more detail in the above-cited French patent application No. 97.11814.
In another embodiment, the [M(0), Mxe2x80x2(0)] polymetallic particles are obtained by a so-called xe2x80x9cpolyolxe2x80x9d process, by dissolution or by placing a precursor of each of the metals M and Mxe2x80x2 in suspension in a polyol, then heating under reflux. Such a process is described, for example, in L. K. Kurihara et al., (Nanostructured Materials, Vol. 5, No. 6, 607-613, 1995), or in L. K. Kurihara et al., (J. Mat. Res., Vol. 10, No. 6, June 1995), as cited above.
In another embodiment, the [M(0), Mxe2x80x2(0)] polymetallic particles are obtained by reduction of a mixture of salts by LiAlH4. In this case, the hydride can be the source of Al as the element Mxe2x80x2. Such a process is described in the above-cited J. A. Haber et al., (Advanced Materials, 1996, 8, No. 2).
In another embodiment, the [M(0), Mxe2x80x2(0)] polymetallic particles are obtained by the reduction of a mixture of salts of the metals M and Mxe2x80x2 by a borohydride, followed by a possible heat treatment, according to a process described, for example, in S. T. Schwab et al. (Materials Science and Engineering, A204 (1995), 197-200). The borohydride is preferably a lithium trialkylborohydride.
The [M(0), Mxe2x80x2(0)] polymetallic particles can likewise be obtained in the form of polymetallic colloids by the formation of inverse micelles, as described particularly in U.S. Pat. No. 5,147,841.
The [M(0), Mxe2x80x2(0)] polymetallic particles can likewise be obtained by the process described in the patent DE-A-44 43 705. This process permits obtaining in colloidal form, particles associating a metal of group VIII and a metal of group Ib.
When the [M(0), Mxe2x80x2(0)] particles are obtained in the form of a suspension in a solvent, they can be subjected to oxidation directly in the solvent. They can also be extracted from the solvent in which they are obtained, to be utilized in the form of a powder.
The process of the invention enables a material to be obtained in the form of particles in which at least 80% of the particles have a mean dimension less than 10 nm, the said particles having a concentration of M(0) higher at the surface of the particle than at the center of the particle.
The particles of the invention can advantageously be used as a catalyst, particularly for hydrogenation reactions or coupling reactions. The [M(0), Mxe2x80x2(0)] polymetallic particles in general themselves possess catalytic properties. However, the inventors have observed that a controlled oxidation of these particles, having as a result the partial or total oxidation of the element Mxe2x80x2 and a possible oxidation of the element M limited to at most 10% by number of atoms of M, enabled a material to be obtained having clearly improved catalytic properties. Indeed, the presence of element Mxe2x80x2 in the oxidized form brings about a reduction of the particle size, and this increases the specific surface of element M(0) active in catalysis. Furthermore, the presence of the element Mxe2x80x2 in the oxidized form reduces the tendency toward agglomeration of the M(0) particles, and this increases the life of the catalyst. Lastly, the presence of the element Mxe2x80x2 in the oxidized form permits a better attachment of the material according to the invention to a support, when the material according to the invention is used as a catalyst carried by a solid support such as, for example, alumina, silica, rutile, cordierite or carbon.